Foamed resinous structures are useful in a wide variety of applications such as thermal insulation, in cushions, as packaging, and as adsorbents. Extruded foams are generally made by melting a polymer together with any desired additives to create a polymer melt. A blowing agent is mixed with the polymer melt at an appropriate temperature and pressure to produce a foamable gel mixture. The foamable gel mixture is then cooled and extruded into a zone of reduced pressure, which results in a foaming of the gel and the formation of the desired extruded foam product. As will be appreciated, the relative quantities of the polymer(s), blowing agent(s), and additives, as well as the temperature and manner in which the pressure is reduced will tend to affect the qualities and properties of the resulting foam product.
Traditional blowing agents used for extruded foam products include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). One of the advantages of both CFC and HCFC blowing agents is their high solubility in a polymer melt during the manufacturing process. Higher blowing agent solubility promotes a reduction in viscosity when the blowing agent is mixed with the polymer melt. In turn, lower viscosity leads to lower energy requirements for mixing. On the other hand, a major disadvantage to these traditional blowing agents is that an increasing number of governments worldwide have mandated the elimination of CFC and HCFC blowing agents due to growing environmental concerns. CFCs, and many other halocarbons, have come to be recognized as serious global environmental threats due to their ability to cause stratospheric ozone depletion and global warming. The ozone depletion and global warming impact of chemicals such as CFCs and HCFCs are measured by the ozone depletion potential (ODP) and global warming potential (GWP) respectively.
In view of the mandatory phase out of blowing agents with a high ODP and a high GWP, there has been a movement to replace the conventional blowing agents with more environmentally friendly blowing agents, such as hydrofluorocarbons (HFCs) and CO2, in insulating foam applications. Although HCFCs provide a superior thermal barrier compared to HFC and CO2, the chlorine present in the HCFCs possesses an ozone depletion potential. Additionally, over time, the chlorofluorocarbon gas phase remaining in the foam is released into the atmosphere, thereby reducing the insulative value of the foam and potentially further contributing to the global warming potential. In addition, each of the “non-conventional” blowing agents leads to a different cell size and morphology, depending on the particular blowing agent chosen. Additionally, the cell sizes of the foams produced by these generally environmentally friendly blowing agents are too small to provide an acceptable insulative value to the foamed product and generally results in a higher density and a more costly product.
In addition, the water vapor permeability of the foams produced with HCFCs typically have a water vapor permeability of 1.0 or less. Desirably, the water vapor permeability of extruded foam boards is greater than 1.0 perm inch. Improving the water vapor permeability in extruded foam boards is becoming an important factor in building construction design and applications. The water vapor permeability of extruded foam boards is an important factor in inhibiting the potential for condensation as well as mold and fungal growth on the foamed boards and in the wall system in which the foam boards are used. When water, dust, and other microbial nutrients contaminate the foam board, they provide a support medium for the growth of bacteria, fungi, and/or mold in and on the foamed board. Bacterial, fungal, and mold growth may cause odor, discoloration, and/or product deterioration. Previous attempts to eliminate mold growth have been focused on limiting the intrusion of liquid water and the condensation of water vapor within the wall assembly.
Despite previous attempts to reduce the ODP and GWP, there remains a need in the art to achieve an extruded polymer foam that has an improved water vapor permeability when non-HCFC blowing agents are used, that maintains the positive physical properties of conventional extruded polystyrene foams, that provides a foam product with increased insulation value (R-value), and that meets the stringent requirements for a reduction in the global warming potential and ozone depletion potential.